2002A&A...384.1054L

Using the Plateau de Bure Interferometer and IRAM 30 m Telescope, we observed λ2-3 mm absorption lines of CS, SO, SO₂, H₂S and HCS⁺ from some of the diffuse clouds which occult our well-studied sample of compact extragalactic mm-wave continuum sources. Our observations of SO, H₂S and HCS⁺ represent the first detections of these species in diffuse clouds; SO₂ was not detected at all. We find a typical value N(CS)≃0.5-2.0x10¹²cm^–2 which is actually much smaller than the values derived previously, along very different lines of sight, from interpretation of CS J = 2-1 emission lines. CS forms somewhat sluggishly and is occasionally absent even in features with appreciable N(HCO⁺). But for lines of sight where CS is found, N(CS)/N(HCO⁺)≃2±1 or X(CS)≃4x10^–9. N(CS) correlates well and varies about linearly with N(HCN) (N(CS)/N(HCN)=0.7±0.3) though, again, lines of sight with appreciable N(HCN) occasionally lack CS. CS correlates well with H₂S (N(CS)/N(H₂S)=6±1) and marginally with SO (N(CS)/N(SO)=1.7±0.8). For the two high column density features observed toward 3C 111 we find N(CS)/N(HCS⁺)=13.3±1.0. CS can easily be shown to form from the observed amounts of HCS⁺ via electron recombination in cool, quiescent gas but the obvious gas-phase routes to formation of HCS⁺ fail by factors of 25 or more.